Impact of massive binary star and cosmic evolution on gravitational wave observations I: black hole–neutron star mergers

Abstract

Mergers of black hole-neutron star (BHNS) binaries have now been observed by GW detectors with the recent announcement of GW200105 and GW200115. Such observations not only provide confirmation that these systems exist, but will also give unique insights into the death of massive stars, the evolution of binary systems and their possible association with gamma-ray bursts, r-process enrichment and kilonovae. Here we perform binary population synthesis of isolated BHNS systems in order to present their merger rate and characteristics for ground-based GW observatories. We present the results for 420 different model permutations that explore key uncertainties in our assumptions about massive binary star evolution (e.g. mass transfer, common-envelope evolution, supernovae), and the metallicity-specific star formation rate density, and characterize their relative impacts on our predictions. We find intrinsic local BHNS merger rates spanning $\mathcal {R}_{\rm {m}}^0 \approx$ 4–830 $\, \rm {Gpc}^{-3}$ $\, \rm {yr}^{-1}$ for our full range of assumptions. This encompasses the rate inferred from recent BHNS GW detections, and would yield detection rates of $\mathcal {R}_{\rm {det}} \approx 1$–180$\, \rm {yr}^{-1}$ for a GW network consisting of LIGO, Virgo and KAGRA at design sensitivity. We find that the binary evolution and metallicity-specific star formation rate density each impact the predicted merger rates by order $\mathcal {O}(10)$. We also present predictions for the GW detected BHNS merger properties and find that all 420 model variations predict that $\lesssim 5{{\ \rm per\ cent}}$ of the BHNS mergers have BH masses $m_{\rm {BH}} \gtrsim 18\, \rm {M}_{\odot }$, total masses $m_{\rm {tot}} \gtrsim 20\, \rm {M}_{\odot }$, chirp masses ${\mathcal {M}}_{\rm {c}} \gtrsim 5.5\, \rm {M}_{\odot }$, mass ratios qf ≳ 12 or qf ≲ 2. Moreover, we find that massive NSs with $m_{\rm {NS}} > 2\, \rm {M}_{\odot }$ are expected to be commonly detected in BHNS mergers in almost all our model variations. Finally, a wide range of $\sim 0{{\ \rm per\ cent}}$–$70{{\ \rm per\ cent}}$ of the BHNS mergers are predicted to eject mass during the merger. Our results highlight the importance of considering variations in binary evolution and cosmological models when predicting, and eventually evaluating, populations of BHNS mergers.

Linking the rates of neutron star binaries and short gamma-ray bursts

“…is the probability of a successful jet launch in an NSBH merger (i.e., a Heaviside step function that evaluates to 0 when r disrupt > r ISCO ), and π(m NS , m BH , χ BH ) is the astrophysical distribution of neutron star and black hole masses and black hole spins of the components that participate in NSBH mergers. We calculate f s,BNS and f s,NSBH using the above prescription and a suite of results from Broekgaarden et al [28] that implements different population-synthesis prescriptions for the binary evolution physics-see that Paper, in particular their Tables 1 and 2, for a description of each of the models. From these various models, we take the mass distribution of neutron stars and black holes that participate in NSBH and BNS mergers and the orbital period before the second binary component explodes as a supernova, the latter informing the spin distribution of black holes that participate in NSBH mergers [28,44].…”

“…We calculate f s,BNS and f s,NSBH using the above prescription and a suite of results from Broekgaarden et al [28] that implements different population-synthesis prescriptions for the binary evolution physics-see that Paper, in particular their Tables 1 and 2, for a description of each of the models. From these various models, we take the mass distribution of neutron stars and black holes that participate in NSBH and BNS mergers and the orbital period before the second binary component explodes as a supernova, the latter informing the spin distribution of black holes that participate in NSBH mergers [28,44]. We also marginalize over the unknown equation of state.…”

“…In particular, the rapid supernovae model (Rapid SNe), fixed supernovae kick (fixed SNe kick), no pulsational pair instability supernovae (No PISN), fixed common envelope efficiency (Fixed CE efficiency), unstable case BB mass transfer, and fixed mass transfer efficiency. For a full description of these simulations we refer the Reader to [28]. In general, the different models predict that anywhere between ≈ 0.5−10% of NSBH mergers disrupt sufficient matter to launch a jet accounting for the unknown equation of state.…”

“…If we assume that GW200105 and GW200115 are representative of the NSBH population, their merger rate is constrained to 45 +75 −33 Gpc −3 yr −1 , and if we account for a broader NSBH population the merger rate is constrained to 130 +112 −69 Gpc −3 yr −1 [16]. These merger rates suggest that GW200105 and GW200115 are likely to have formed in isolated binaries or young clusters [e.g., [25][26][27][28][29]. Independently, observations of BNS coalescences with aLIGO and Virgo have constrained the rate of BNS mergers in the local Universe to 320 +490 −240 Gpc −3 yr −1 [30] 1 .…”

Neutron star black hole binaries in LIGO/Virgo O3b run were formed from Population I/II binaries

“…Mandel & Broekgaarden (2021) gave us a great summary on merger rates of compact object binaries, i.e., NS-NSs, BH-BHs and NS-BHs for GW observations and various formation channels. Using metallicity-specific star formation rate density prescriptions (Broekgaarden et al 2021b) in an isolated binary evolution scenario, Broekgaarden & Berger (2021) concluded that the observed two NS-BHs can be explained from this formation channel, and gave some constraint on the common envelope efficiency and the natal kick velocities of supernovae by treating BH-BHs and NS-NSs simultaneously. Taking the spin direction of BH of GW200115 into account, Fragione et al (2021) discussed this NS-BH in the evolution of isolated massive binary stars and effects 2021) have predicted that 16-40% events of short-duration gamma-ray burst will be hostless.…”